In the digital coherent transmission, signals modulated by existing digital modulation can be allocated to each of two orthogonal polarization components (V, H). For example, in the case where each of the two polarization components V and H includes an I (in-phase) channel and a Q (quadrature) channel, polarization multiplexed light including a total of four channels (HI, HQ, VI and VQ) can be transmitted through an optical fiber.
On the receiving side, the polarization multiplexed light arriving through the optical fiber is demultiplexed for each polarization, and the I channel component and the Q channel component are separated from each polarized light to detect four channel (HI, HQ, VI and VQ) signals. In general, a digital coherent receiver includes a coherent receiving circuit that detects four channel components (HI, HQ, VI, and VQ) from the above-described polarization multiplexed light, and a digital signal processor that converts the four channel signals into digital signals and performs various kinds of signal processing including phase compensation, skew compensation, demodulation, and the like.
In particular, it is considered that skew between channel signals generated inside the receiver may be caused by unequal optical path lengths in the receiver, unequal physical lengths of electrical wiring connecting an optoelectronic converter and a DSP, variations in characteristics of the optoelectronic converters and AD (Analog-to-Digital) converters, and the likes. It is almost impossible to physically eliminate such inter-signal skew. However, skew between the above-mentioned four channel signals would affect signal reproduction by digital signal processing to impair excellent characteristics as a coherent receiver. There have been proposed various techniques for compensating skews by digital signal processing.
For instance, PTL 1 discloses a digital coherent receiver that detects skew between channel signals and controls a skew adjustment value of each channel signal based on detected skew values. PTL 2 discloses a polarization multiplexing transponder that detects skew between parallel signals by comparing predetermined amplitude patterns imprinted to each of the parallel signals to perform skew compensation.